Photoresistor

ABSTRACT

A photoresistor includes a first electrode layer, a photosensitive material layer, and a second electrode layer. The first electrode layer, photosensitive material layer and second electrode layer are stacked with each other. The first electrode layer is located on a first surface of the photosensitive material layer. The second electrode layer is located on a second surface of the photosensitive material layer. The first surface and second surface of the photosensitive material layer are opposite to each other. The first electrode layer includes a carbon nanotube film structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201310042597.1, filed on Feb. 4, 2013 inthe China Intellectual Property Office. This application is also relatedto the applications entitled, “METHOD FOR DETECTING POLARIZED LIGHT”,filed Apr. 8, 2013 Ser. No. 13/858,728 and entitled “POLARIZED LIGHTDETECTION SYSTEM”, filed Apr. 8, 2013 Ser. No. 13/858,735. Disclosuresof the above-identified applications are incorporated herein byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to photoresistors and, more particularlyto a photoresistor incorporating carbon nanotubes.

2. Description of Related Art

A photoresistor or light dependent resistor is a resistor whoseresistance decreases with increasing incident light intensity.

In related arts, a photoresistor includes an active semiconductor layerdeposited on an insulating substrate, and two metal contacts placedseparately on the exposed area of the semiconductor layer. Within thephotoresistor structure, the resistance of the semiconductor materialitself is a key issue. To ensure the resistance changes resulting fromthe light dominate, the resistance of the two metal contacts isminimized. To achieve this, the area between the two metal contacts isin the form of a zigzag or interdigital pattern. This keeps the distancebetween the two metal contacts small, which reduces the resistance andenhances the carriers gain.

However, because the metal contacts are impenetratable by light, theexposed area of the semiconductor is still small and the sensitivity ofthe photoresistor to the weak light is low. Furthermore, thephotoresistor cannot be used to detect the polarized light withoutexternal polarizing elements.

What is needed, therefore, is to provide a photoresistor which is moresensitive to the weak light and can also be used to detect polarizedlight without external polarizing element.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, the emphasis instead being placed upon clearlyillustrating the principles of the present embodiments.

FIG. 1 is a schematic diagram of a photoresistor according to oneembodiment.

FIG. 2 is an exploded view of a carbon nanotube film structure used inthe photoresistor of FIG. 1.

FIG. 3 is an exploded view of another carbon nanotube film structureused in the photoresistor of FIG. 1.

FIG. 4 is a scanning electron microscopy (SEM) picture of a single-layercarbon nanotube drawn film used in the photoresistor of FIG. 1.

FIG. 5 is schematic diagram of a polarized light detection systemincorporating the photoresistor of FIG. 1.

FIG. 6 is flowchart of method for detecting a polarized light using thepolarized light detection system of FIG. 5.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone.

Referring to FIG. 1, a photoresistor 10 of one embodiment includes afirst electrode layer 102, a photosensitive material layer 104, and asecond electrode layer 106. The first electrode layer 102,photosensitive material layer 104 and second electrode layer 106 arestacked with each other. The photosensitive material layer 104 has afirst surface 1042, a second surface 1044 opposite to the first surface1042, and at least one side surface connecting the first surface 1042 tothe second surface 1044. The first electrode layer 102 is attached tothe first surface 1042 of the photosensitive material layer 104; and thesecond electrode layer 106 is attached to the second surface 1044 of thephotosensitive material layer 104. The first electrode layer 102 andsecond electrode layer 106 are electrically contacted to thephotosensitive material layer 104.

In one embodiment, the first electrode layer 102 and second electrodelayer 106 cover the entire first surface 1042 and the entire secondsurface 1044, respectively. In another embodiment, the first electrodelayer 102 and second electrode layer 106 cover parts of the firstsurface 1042 and parts of the second surface 1044, respectively.

According to one embodiment, the photoresistor 10 further includes afirst lead-out electrode 110 and a second lead-out electrode 108. Thefirst lead-out electrode 110 and second lead-out electrode 108 areseparately configured. The first lead-out electrode 110 is electricallycontacted to the first electrode layer 102, and the second lead-outelectrode 108 is electrically contacted to the second electrode layer106. Furthermore, the first lead-out electrode 110 is isolated from thephotosensitive material layer 104 and second electrode layer 106. Thesecond lead-out electrode 108 is isolated from the photosensitivematerial layer 104 and first electrode layer 102.

According to another embodiment, the photoresistor 10 includes aninsulating substrate 112. The substrate 112 is used to support the wholephotoresistor 10. The second electrode layer 106 and first lead-outelectrode 110 are separately located on a surface of the substrate 112.The second lead-out electrode 108 and photosensitive material layer 104are separately located on a surface of the second electrode layer 106.The first electrode layer 102 is simultaneously located on the firstsurface 1042 of the photosensitive material layer 104 and a surface ofthe first lead-out electrode 110.

The photosensitive material layer 104 is made of a semiconductor. Thesemiconductor can be cadmium sulphide, lead sulphide, aluminiumsulphide, bismuth sulphide, germanium, selenium, silicon, or galliumarsenide. In one embodiment, the photosensitive material layer 104 ismade of cadmium sulphide.

The thickness of the photosensitive material layer 104 is in a rangefrom about 5 microns to about 500 microns. In one embodiment, thethickness of the photosensitive material layer 104 ranges from about 5microns to about 150 microns. In another embodiment, the thickness ofthe photosensitive material layer 104 ranges from about 5 microns toabout 50 microns. When the photosensitive material layer 104 has asmaller thickness, the carriers excited by photons need to step over asmaller gap between the two electrode layers. The light sensitivity ofthe photoresistor 10 is improved because fewer carriers are loss.

Referring to FIG. 2, the first electrode layer 102 is a transparentconductive electrode, which includes a carbon nanotube film structure1020. In one embodiment, the first electrode layer 102 consists of thecarbon nanotube film structure 1020.

The carbon nanotube film structure 1020 is a flexible and free-standingstructure. The term “free-standing structure” can be defined as astructure that does not have to be supported by a substrate and cansustain the weight itself when it is hoisted by a portion thereofwithout tearing. The thickness of the carbon nanotube film structure1020 is in a range from about 1 nanometer to about 50 microns. In oneembodiment, the thickness of the carbon nanotube film structure 1020ranges from about 0.5 microns to about 5 microns. The carbon nanotubefilm structure 1020 includes a plurality of evenly distributed carbonnanotubes 1022. In one embodiment, the carbon nanotube film structure1020 consists of the plurality of carbon nanotubes 1022. The adjacentcarbon nanotubes 1022 in the carbon nanotube film structure 1020 arejoined by van der Waals attractive force therebetween. The plurality ofcarbon nanotubes 1022 in the carbon nanotube film structure 1020 issubstantially aligned along a single preferred direction. The carbonnanotubes 1022 can be single-walled carbon nanotubes, double-walledcarbon nanotubes, or multi-walled carbon nanotubes.

Referring to FIG. 3 to FIG. 4, the carbon nanotube film structure 1020includes at least one carbon nanotube drawn film 1021. The carbonnanotube drawn film 1021 is a free-standing structure including aplurality of successive and oriented carbon nanotubes 1022. The carbonnanotubes 1022 in the carbon nanotube drawn film 1021 are arrangedsubstantially parallel to a surface of the carbon nanotube drawn film1021. The plurality of carbon nanotubes 1022 are joined end to end byvan der Waals attractive force therebetween. If the carbon nanotube filmstructure 1020 consists of a plurality of carbon nanotube drawn films1021 stacked with each other, all of the carbon nanotubes 1022 in thecarbon nanotube drawn films 1021 are substantially aligned along asingle preferred direction.

The carbon nanotube drawn film 1021 has a light transmittance rangesfrom about 80% to about 90%. If the carbon nanotube film structure 1020consists of a plurality of carbon nanotube drawn films 1021 stacked witheach other, a light transmittance of the carbon nanotube film structure1020 will decrease gradually. According to one embodiment, the carbonnanotube film structure 1020 consists of two carbon nanotube drawn films1021, and the light transmittance decreases to about 70% to about 80%.Thus, the number of the carbon nanotube drawn films 1021 in the carbonnanotube film structure 1020 is less than 5. In one embodiment, thenumber of the carbon nanotube drawn films 1021 in the carbon nanotubefilm structure 1020 is less than 3.

To improve the light transmittance of the carbon nanotube drawn film1021, a laser treatment for reducing the thickness of the carbonnanotube drawn film 1021 can be applied. According to one embodiment,the light transmittance of the carbon nanotube drawn film 1021 increasesto 95% or greater than 95% after the laser treatment. In one embodiment,a power density of a laser is greater than 0.1×10⁴ W/m², a diameter ofan irradiating pattern of the laser approximately ranges from 1 micronto 5 millimeters, wherein a time of laser irradiation is less than 1.8s.

The carbon nanotube drawn film 1021 can be formed by the steps of:

(a) providing an array of carbon nanotubes or a super-aligned array ofcarbon nanotubes;

(b) selecting a plurality of carbon nanotube segments having apredetermined width from the array of carbon nanotubes; and

(c) pulling the carbon nanotube segments at an even speed to form acarbon nanotube drawn film.

In step (a), the super-aligned array of carbon nanotubes can be formedby substeps of:

(a1) providing a substantially flat and smooth substrate;

(a2) forming a catalyst layer on the substrate;

(a3) annealing the substrate with the catalyst layer in air at atemperature ranging from 700° C. to 900° C. for about 30 minutes to 90minutes;

(a4) heating the substrate with the catalyst layer at a temperatureranging from 500° C. to 740° C. in a furnace in protective gas; and

(a5) supplying a carbon source gas to the furnace for about 5 minutes to30 minutes and growing a super-aligned array of carbon nanotubes fromthe substrate.

The super-aligned array of carbon nanotubes can be approximately 50microns to 900 microns in height, and includes a plurality of carbonnanotubes parallel to each other and substantially perpendicular to thesubstrate. The super-aligned array of carbon nanotubes formed under theabove conditions is essentially free of impurities, such as carbonaceousor residual catalyst particles. The carbon nanotubes in thesuper-aligned array are closely packed together by van der Waalsattractive force.

In step (b), the carbon nanotube segments having a predetermined widthcan be selected using an adhesive tape to contact with the super-alignedarray.

In step (c), the pulling direction is substantially perpendicular to thegrowing direction of the super-aligned array of carbon nanotubes.Specifically, during the pulling process, as the initial carbon nanotubesegments are drawn out, other carbon nanotube segments are also drawnout end to end due to the van der Waals attractive force between ends ofadjacent segments. This process of drawing ensures a successive carbonnanotube drawn film having a predetermined width can be formed.

The second electrode layer 106 can be the same as the first electrodelayer, which consists of a carbon nanotube film structure 1020. Thesecond electrode layer 106 can also consist of a metal film or an ITOfilm. If the second electrode layer 106 consists of the carbon nanotubefilm structure 1020, the photoresistor 10 can be a flexible structure asa whole. If the second electrode layer 106 consists of the metal film,the second electrode layer 106 can reflect an incident light and thenimprove the light sensitivity of the photoresistor 10. The metal filmcan be made of aluminum, copper, silver, gold, or their alloys. In oneembodiment, the metal film is a copper film.

Both the first lead-out electrode 110 and second lead-out electrode 108can be made of metals such as silver, copper, and aluminum. The firstlead-out electrode 110 and second lead-out electrode 108 can also bemade of ITO or carbon nanotubes. The first lead-out electrode 110 andsecond lead-out electrode 108 can be but not limited to wire-shaped,stripe-shaped, or plate-shaped. In one embodiment, both the firstlead-out electrode 110 and second lead-out electrode 108 are aluminumwires. In one embodiment, the first lead-out electrode 110 is located onone end of the first electrode layer 102 along the single preferreddirection of the carbon nanotubes 1022. In another embodiment, thesecond electrode layer 106 consists of a carbon nanotube film structure1020, and the second lead-out electrode 108 is located on one end of thesecond electrode layer 106 along the single preferred direction of thecarbon nanotubes 1022.

The insulating substrate 112 can be made of flexible polymers such aspolyethylene terephthalate (PET), polymethyl methacrylate (PMMA),polycarbonate (PC), and polyethylene (PE). The insulating substrate 112can also be made of glasses, silicon, silicon oxide, silicon carbide, orother rigid materials. The thickness of the insulating substrate 112 isnot limited. In one embodiment, the insulating substrate 112 is a PETfilm with a thickness of about 200 microns.

The transparent carbon nanotube film structure 1020 used as the firstelectrode layer 102 of the photoresistor 10 can increase the exposedarea of the photosensitive material layer 104 to light and then increasethe light received by the photoresistor 10, and thus improve the lightsensitivity of the photoresistor 10. Furthermore, because the carbonnanotubes 1022 are substantially aligned along a single direction, thecarbon nanotube film structure 1020 is anisotropic in some propertiessuch as the light transmittance. The transmittance of the carbonnanotube film structure 1020 to a polarized light whose polarizationdirection is parallel to the single direction of the carbon nanotubes1022 is smaller than the transmittance to a polarized light whosepolarization direction is perpendicular to the single direction of thecarbon nanotubes 1022. The carbon nanotube film structure 1020 can thusalso be used as a polarizing element in the photoresistor 10. Therefore,the photoresistor 10 can be used to detect the polarized light.

Referring to FIG. 5, a polarized light detection system 100 according toone embodiment includes a photoresistor 10, a power source 114, and adetection apparatus 116. The photoresistor 10, power source 114 anddetection apparatus 116 are electrically connected with wires to form agalvanic circle.

The power source 114 is a direct current power source with a voltagebetween 0.01 V to 2 V.

The detection apparatus 116 includes a current detection device such asan amperemeter to detect a fluctuation of the photoresistor 10. Thedetection apparatus 116 can further include a computer analysis systemto analyze the polarization information of an incident light.

Referring to FIG. 6, a method for detecting polarized light of oneembodiment includes steps of:

S1, providing a polarized light detection system 100 including aphotoresistor 10, a power source 114, and a detection apparatus 116,wherein the photoresistor 10 includes a first electrode layer 102 and aphotosensitive material layer 104, the detection apparatus 116 includesa current detection device and a computer analysis system;

S2, irradiating an incident light onto a surface of the photoresistor10;

S3, identifying polarization information of the incident light by thephotoresistor 10;

S4, detecting current change in the photoresistor 10 by the currentdetection device; and

S5, analyzing the polarization information of the incident light by thecomputer analysis system.

In step S1, the photoresistor 10, power source 114 and detectionapparatus 116 are electrically connected with wires to form a galvaniccircle.

In step S2, the incident light is irradiated onto a surface of the firstelectrode layer 102.

In step S3, the identification of polarization information of theincident light is executed through adjusting the position and directionrelationships between the photoresistor 10 and light source of theincident light. One way of adjusting the position and directionrelationships is to rotate one of the photoresistor 10 and light sourcewhile maintaining the distance unchanged.

In step S4, the instant value of current relates to the transmittance tothe incident light of the first electrode layer 102. Smaller currentvalue responds to smaller transmittance of the incident light.Therefore, when the current in the photoresistor 10 decreases to aminimum value, the transmittance to the incident light of the firstelectrode layer 102 goes to a minimum value also. This indicates that apolarization direction of the incident light is parallel to the singledirection along which the carbon nanotubes 1022 in the carbon nanotubefilm structure 1020 aligned in the first electrode layer 102.Alternatively, when the current in the photoresistor 10 increases to amaximum value, the transmittance to the incident light of the firstelectrode layer 102 goes to a maximum value too. This indicates that apolarization direction of the incident light is perpendicular to thesingle direction along which the carbon nanotubes 1022 in the carbonnanotube film structure 1020 aligned in the first electrode layer 102.

The method as disclosed in FIG. 6 can be used to detect polarized light,and also be used to detect weak light, mainly because of thetransparency and anisotropy features of the carbon nanotube filmstructure 1020 in the first electrode layer 102 of the photoresistor 10.

It is to be understood that the above-described embodiment is intendedto illustrate rather than limit the disclosure. Variations may be madeto the embodiment without departing from the spirit of the disclosure asclaimed. The above-described embodiments are intended to illustrate thescope of the disclosure and not restricted to the scope of thedisclosure.

It is also to be understood that the above description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A photoresistor comprising: a photosensitivematerial layer comprising a first surface and a second surface oppositeto each other; a first electrode layer, located on the first surface,comprising a carbon nanotube film structure, wherein the carbon nanotubefilm structure consists of a plurality of carbon nanotubes substantiallyaligned along a single preferred direction; and a second electrode layerlocated on the second surface.
 2. The photoresistor as claimed in claim1, wherein the carbon nanotube film structure is a flexible andfree-standing structure.
 3. The photoresistor as claimed in claim 1,wherein the plurality of carbon nanotubes are joined by van der Waalsattractive force therebetween.
 4. The photoresistor as claimed in claim3, further comprising a first lead-out electrode and a second lead-outelectrode, the first lead-out electrode and second lead-out electrodeare spaced from each other; wherein the first lead-out electrode iselectrically connected to the first electrode layer, and the secondlead-out electrode is electrically connected to the second electrodelayer.
 5. The photoresistor as claimed in claim 4, wherein the firstlead-out electrode is located on one end of the first electrode layeralong the single preferred direction.
 6. The photoresistor as claimed inclaim 4, further comprising an insulating substrate with a surface;wherein the second electrode layer and the first lead-out electrode areseparately located on the surface of the insulating substrate.
 7. Thephotoresistor as claimed in claim 1, wherein the carbon nanotube filmstructure comprises at least one carbon nanotube drawn film.
 8. Thephotoresistor as claimed in claim 7, wherein the at least one carbonnanotube drawn film comprises a plurality of successive and orientedcarbon nanotubes joined end to end by van der Waals attractive forcetherebetween.
 9. The photoresistor as claimed in claim 7, wherein the atleast one carbon nanotube drawn film has a light transmittance rangingfrom about 80% to about 90%.
 10. The photoresistor as claimed in claim7, wherein the at least one carbon nanotube drawn film comprises twocarbon nanotube drawn films stacked with each other and the carbonnanotube film structure has a light transmittance ranging from about 70%to about 80%, the plurality of carbon nanotubes in the two carbonnanotube drawn films are substantially aligned along a single preferreddirection.
 11. The photoresistor as claimed in claim 7, wherein the atleast one carbon nanotube drawn film is treated by a laser and has alight transmittance greater than 95%.
 12. The photoresistor as claimedin claim 1, wherein the second electrode layer comprises a metal film.13. The photoresistor as claimed in claim 12, wherein the metal film ismade of aluminum, copper, silver, gold, or any alloys thereof.
 14. Thephotoresistor as claimed in claim 1, wherein the first electrode layercovers the entire first surface of the photosensitive material layer,and the second electrode layer covers the entire second surface of thephotosensitive material layer.
 15. The photoresistor as claimed in claim1, wherein the photosensitive material layer comprises a semiconductor,and the semiconductor is selected from the group consisting of cadmiumsulphide, lead sulphide, aluminium sulphide, bismuth sulphide,germanium, selenium, silicon, and gallium arsenide.
 16. Thephotoresistor as claimed in claim 1, wherein the photosensitive materiallayer has a thickness ranging from about 5 microns to about 500 microns.17. The photoresistor as claimed in claim 16, wherein the photosensitivematerial layer has a thickness ranging from about 5 microns to about 50microns.
 18. A photoresistor comprising: a photosensitive material layercomprising a first surface and a second surface opposite to each other;a first electrode layer located on the first surface; and a secondelectrode layer located on the second surface; wherein each of the firstelectrode layer and the second electrode layer comprises a carbonnanotube film structure consisting of a plurality of carbon nanotubessubstantially aligned along a single preferred direction.
 19. Thephotoresistor as claimed in claim 18, wherein the carbon nanotube filmstructure has a light transmittance ranging from about 70% to about 80%.20. The photoresistor as claimed in claim 18, wherein the carbonnanotube film structure comprises less than 5 carbon nanotube drawnfilms.